1. Field of the Invention
This invention relates to a focus detecting device for detecting an in-focus state by using an image signal produced from an image sensor.
2. Description of the Related Art
In the field of apparatuses for forming electrical image signals representing images of objects to be pictured by means of CCD image sensors such as video cameras, etc., there has been known an automatic focusing method of adjusting the focal point of an optical system by evaluating the sharpness of the object's image from the image signal. The conventional focus detecting circuit for carrying out this method is arranged as shown in FIG. 1 of the accompanying drawings. An image signal from an image sensor 10 is differentiated by a differentiating circuit 12. The absolute value of the differentiated signal is then obtained by an absolute value circuit 14. The peak value of the differentiated absolute value signal is detected by a peak detecting circuit 16. The peak value thus obtained is evaluated by an evaluation circuit 18. This method utilizes the fact that the larger the peak value is, the closer the optical system to an in-focus point and that the peak value reaches the largest value at the in-focus point. In another method which is similar to this, a high frequency component is extracted from the image signal and the sharpness of the object image is judged from the amplitude of the extracted component.
With regard to the focus detecting devices of this kind, a method called "a hill climbing servo method" which is described in detail, for example, in "NHK GIJUTSU KENKYU," 1965, Vol. 17, No. 1, Whole No. 86, page 21 to 37, is well known among others.
The focus detecting methods of the above stated kind, however, give an evaluated or estimated sharpness value (peak value or amplitude value of a high frequency component) which fluctuates with the kind or contrast of the object. They are, therefore, incapable of making absolutely reliable focus detection. To ensure a reliable result of such focus detection, therefore, it is necessary to make a final determination after evaluated values obtained from consecutive images (fields or frames) are compared with each other.
To remove the shortcoming of the above stated focus detecting methods, there has been proposed another method, which is disclosed in U.S. patent application Ser. No. 922,740, filed Oct. 24, 1986, which is assigned to the same assignee as that of the present application. In the case of this method, importance is attached to image parts obtained from the edge parts, such as the contour, etc., of an object. An estimated value of sharpness of the object is obtained from a ratio between the brightness gradient and brightness difference of the edge parts of the image. This estimated or evaluated value absolutely represents the focused state of the optical system. The method obviates the necessity of comparing the evaluated values obtained from consecutive images and permits focus detection from a single piece of image. The same method is also employed in a device disclosed in U.S. patent application Ser. No. 046,252, filed May 5, 1987, and assigned also to the same assignee as that of the present application.
However, in detecting edge width, the range within which this method is capable of making accurate detection is limited to a relatively narrow range around an in-focus point due to a problem relative to an evaluation value computing circuit. Therefore, the automatic focusing device likely comes out of control while the optical system is out of focus. Meanwhile, in the case of the conventional focus detecting method of FIG. 1, the automatic focusing device is advantageous in controlling the optical system toward its in-focus point in its out-of-focus state. However, the device tends to cause hunting or the like in the neighborhood of the in-focus point and is, therefore, incapable of performing sufficiently stable control.
Further, the patent application cited above also discloses another focus detecting device which is arranged to take the advantages of the above stated two different methods and to mutually offset their shortcomings. However, this device includes many things that still remain to be improved.